1. Field of the Invention
The present invention relates generally to an electric power steering apparatus which provides power assist of an electric motor directly to a steering system so as to reduce necessary steering power to be applied by the driver. More particularly, it relates to such an electric power steering apparatus which is capable of providing the power assist even when operation of a sensor such as a motor current detector becomes abnormal.
2. Description of the Related Art
FIG. 4 of the accompanying drawings shows in block diagram the general construction of a control unit 120 of a conventional electric power steering apparatus.
The conventional control unit 120 generally includes a target current calculating section 121, a target current correcting section 122, a feedback (F/B) control section 123, a motor driving section 124, a motor speed calculating section 125, a sensor fault detecting section 126, and a fault indicating section or indicator 127.
The target current calculating section 121 determines a target assist torque on the basis of a steering torque signal Tp supplied from a steering torque sensor 112 and outputs a signal (hereinafter referred to as "target current") IT corresponding to a target current value required for an electric motor 110 to apply the target assist torque to a steering system. The target current IT is supplied to the target current correcting section 122.
The target current correcting section 122 calculates and outputs a signal (hereinafter referred to as "corrected target current") ITH corresponding to a corrected target current value resulting from a correction made to the target current IT on the basis of a steering angle signal 114a supplied from a steering angle sensor 114, a vehicle velocity signal 116a supplied from a vehicle velocity sensor 116, yaw rate signal 118a supplied from a yaw rate sensor 118, and a motor speed signal 125a determined by calculation at a motor speed calculating section 125 to meet the current vehicle running conditions such as the steering condition, vehicle velocity, and yaw rate, and the rotational speed of an electric motor 110. The corrected target current ITH is supplied to the feedback control section 123.
The feedback control section 123 includes an offset calculating part 131 and a PID (proportional and integral and derivative) control part or controller 132. The offset calculating part 131 determines an offset between the corrected target current ITH and a signal (hereinafter referred as "motor current") IM corresponding to a motor current detected by a motor current detecting unit or detector 128, and outputs an offset signal 131a representing the determined offset (ITH-IM). The offset signal 131a is supplied to the PID controller 132. The PID controller 132 applies PID arithmetic processing to the offset signal 131a to generate a drive control signal 132a which controls a current to be supplied to the electric motor 110 so as to render the offset (ITH-IM) zero. The drive control signal 132a is supplied to the motor drive section 124.
The motor drive section 124 includes a PWM (pulse-width modulation) signal generating part or generator 141, a gate drive circuit 142, and a motor drive circuit 143 consisting of four power FETs (field-effect transistors) connected in an H-type bridge. The PWM signal generator 141 generates, on the basis of the drive control signal 132a, a PWM signal 141a for PWM-driving the electric motor 110. The PWM signal 141a is supplied to the gate drive circuit 142. The gate drive circuit 142 drives the gates of the FETs and thereby drives switching of the FETs on the basis of the PWM signal 141a. Thus, the control unit 120 PWM-controls power supplied from a battery power source BAT to the electric motor 110 on the basis of the steering torque Tp detected by the steering torque sensor 112 and thereby control the output power (steering assist force or torque) of the electric motor 110.
The motor speed calculating section 125 calculates a rotational speed of the electric motor 110 on the basis of the motor current IM detected by the motor current detector 128 and a signal (hereinafter referred to as "motor voltage") VM corresponding to a motor voltage detected by a motor voltage detecting unit or detector 129, and outputs a motor speed signal 125a corresponding to the calculated rotational speed of the electric motor 110.
The sensor fault detecting section 126 monitors the steering angle signal 114a, the vehicle velocity signal 116a, the yaw rate signal 118a, the motor speed signal 125a, the motor current IM and the motor voltage VM. When any one of the signals 114a, 116a, 118a, 125a, IM and VM is outside a predetermined range of signal value set in advance for each signal, when any one of the signals 114a, 116a, 118a, 125a, IM and VM is not supplied from the corresponding sensor or detector, or when any one of the signals 114a, 116a, 118a, 125a, IM and VM varies abnormally, the sensor fault detecting section 126 judges the sensor (detector) 114, 116, 118, 125, 128 or 129 to be operating abnormally and outputs a sensor fault detection signal 126a. The sensor fault detecting section 126 is constructed to store detection of a sensor fault in a nonvolatile memory so that the sensor fault detection signal 126a is automatically output when the power is turned on at the next operation the control unit 120. The sensor fault detection signal 126a is supplied to the motor drive section 124 and the fault indicator 127.
Upon receipt of the sensor fault detection signal 126a, the motor drive section 124 stops the PWM signal generating operation or the gate driving operation, or opens contacts of a relay (not shown) disposed between the battery power source BAT and the motor drive circuit 143, thereby stopping operation of the electric motor 110.
The fault indicator 127 is an indicator that upon receipt of the sensor fault detection signal 126a, provides an instantaneous alarm, both visual and audible, of a failure detected in the sensor to thereby indicate that a failure arises in the sensors and the electric power steering apparatus is in the inoperative condition due to the failure in the sensor.
However, in the conventional electric power steering apparatus having the control unit 120 shown in FIG. 4, when the motor current detector 128 provided for the feedback control of operation of the electric motor 110 or another sensor provided to generate a signal for the correction of a steering assist torque in accordance with running conditions of the vehicle becomes anomalous in operation, or when a sensor's operation failure is detected in error, because supply of the steering assist force from the electric motor 110 is suddenly stopped, the steering wheel becomes suddenly heavy, and this may be disconcerting for the driver.
To avoid these difficulties caused by sudden stop of the supply of steering assist force, it may be considered that two sets of sensors are provided so that when one set of sensors becomes faulty, operation of the control unit can be continued based on information from the other set of sensors. Such fail-safe system, however, will increase the number of components needed and the cost of the vehicle.